Placental physiology and fetal programming in ruminants under heat stress

The placenta plays a crucial role in transferring nutrients and oxygen between the dam and fetus during pregnancy. It is highly influenced by environmental conditions, especially stressors such as heat and nutritional deficiencies, which can significantly impact the fetus's long-term health and development. Cattle, especially dairy cows, commonly experience stress during late gestation, which can lead to changes in behavior and physiology, affecting both subsequent milk production and fetal development. Heat stress is one of the most common stressors experienced by mammals, and recent evidence suggests a role in the programming of the dam and fetus. This review explores different hypotheses of fetal programming, including the Barker Hypothesis, which connects early-life malnutrition to metabolic diseases in adulthood, and the Silver-Spoon Hypothesis, which highlights the long-term benefits of optimal prenatal conditions. Furthermore, we consider heat stress programming as it relates to the concept of developmental origins of health and diseases (DOHaD). The DOHaD hypothesis suggests that epigenetic adaptations occur in fetal DNA as a response to environmental influences. The review also emphasizes the role of the mechanism associated with possible epigenetic effects in the placenta, mediating the effects of maternal stress on the fetus, impacting gene expression, placental structure, and nutrient transfer. Understanding these mechanisms is essential for enhancing dairy cattle management and minimizing the adverse effects of environmental stressors on animal health and productivity.